By Tami Moon-Carlson, lead laboratory analyst for the city of Northglenn
You can mark your calendar by it. Every September, the Northglenn Water Treatment Plant receives calls from residents who are experiencing a musty or metallic taste and odor in their drinking water.
After talking with staff, residents are surprised to learn that the change in taste and odor is a result of seasonal mixing in Standley Lake and usually lasts about a month.
Seasonal mixing of a lake is a physical process known as "lake turnover" and happens when lake temperatures become stratified or layered. In the spring, water temperatures in Standley Lake are consistent and the water in the lake mixes easily from the top to bottom.
Over the summer, air and water temperatures begin to rise, algae growth increases and it becomes difficult for sunlight to reach the bottom of the lake. The sunlight can only warm the surface of the water. The colder water, which is heavier, stays at the bottom and does not mix with the warm water on the top.
This causes the lake to stratify, or layer into three distinct sections: The warm top layer (epilimnion), the middle barrier layer (thermocline) and the cold bottom layer (hypolimnion).
In this layered condition, the cold water at the bottom of Standley Lake can't receive the warm, nutrient-rich and oxygen-rich water at the top of the lake.
As the summer moves along, the lack of oxygen at the bottom of the lake causes a condition called anoxia. As the bottom of the lake becomes more and more anoxic, many changes to the sediments and the chemical make-up of the water occur.
Late in September, as air temperatures begin to cool and the breezes pick up, the warm water on the top of the lake starts to cool. Under the right temperature conditions and the right amount of wind, the lake begins to mix again.
This change is very rapid, and usually happens in just a few hours. Daily monitoring of temperature and oxygen in Standley Lake allows us to pinpoint the exact date of turnover. Standley Lake turns over every year around Sept. 20.
How does the lake turning over affect water quality? In early September, as the bottom of Standley Lake becomes increasingly more oxygen-deprived, or anoxic, the sediments at the bottom of the lake begin to release compounds such as iron, manganese, sulfur and organic compounds into the water.
Taste and odor changes are caused by metals and organic matter. The musty taste and odor is caused by decaying plant material in the sediments of the lake. Manganese and iron are common metals found naturally in the environment and even in our bodies.
Our bodies depend on iron and manganese for making blood and helping organs function.
However, in the water and without oxygen, these metals cause a metallic taste, and the manganese can cause the chlorine used to disinfect Northglenn's drinking water to be more apparent.
Shortly after turnover occurs, Northglenn's water will return to normal. However, some residents will notice the taste and odor for one to two weeks after turnover because water treated prior to turnover is still in the distribution system.
To help reduce some of the taste and odor from lake turnover, residents can place a pitcher of water into the refrigerator. The colder temperature and exposure to air can help dissipate some of the taste and odor from the water.
Northglenn's water is safe to drink and meets all drinking water standards set by the Safe Drinking Water Act throughout this process.